Indication of battery condition generally requires testing the battery while drawing high current. For example, the commonly accepted test for specifying battery capability (Cold Cranking Amps) is defined by the Society of Auto Engineers (CCA test J537), wherein battery voltage drop is monitored after discharging at a specified constant high current for 30 seconds. This invention relates generally to battery testers and more particularly to electronic testing instruments that draw very little power from the battery, but derive results as if testing with high power drain.
Electronic battery testers have become accepted in the marketplace because of distinct advantages. In addition to their convenience as hand-held instruments without heavy cables, they are safer because the small currents drawn avoid dangerous sparks. Further, they do not require the battery to be fully charged in order to test nor will electronic battery testers weaken an already low battery. All electronic testers employ essentially similar basic technology to that described in U.S. Pat. No. 4,719,428 issued to Leonard Liebermann, on Jan. 12, 1988, whereby a small periodically switched test current is drawn from the battery. The resulting alternating voltage across the battery terminals is directly proportional to internal resistance. With use, a battery""s internal resistance increases, and hence the observed alternating voltage increases. U.S. Pat. No. 4,719,428 amplifies the alternating voltage, converts it to steady D.C. and subtracts it from the total battery voltage. Battery output voltage drops while a large current is drawn, and that device indicates the expected voltage under these conditions.
Largely because of the SAE rating of batteries for their current capability, electronic battery testers have been developed to test current capability. The current capability is inversely proportional to the already discussed internal resistance. U.S. Pat. No. 4,816,768 discloses a circuit that provides an output that is inversely proportional to resistance and thus is indicative of current capability. That technology employs a feedback amplifier with the battery in the feedback loop. However with feedback technology as the battery resistance increases with use, the test load current is reduced proportionately. Typically, battery resistance increases by a factor 3 or 4 with use, before it needs replacement. Consequently a used battery may be tested with ⅓ or xc2xc reduced load current, compared to testing when new. Battery resistance is unlike a metallic resistor and its observed value can vary with test current. In checking the decline of a battery with use it would be clearly preferable to test with the same test load, rather than the variation introduced with feedback technology.
Broadly speaking, this invention provides a storage battery tester that tests the current capability of a battery, but does so without drawing large currents. As a consequence, no heavy cable leads, heavy contacting clips, nor high heat dissipating resistors are required. It can be hand-held and requires no internal batteries, deriving its power supply from the storage battery under test.
The small test current is drawn from the battery by a switching circuit connected across its terminals and controlled by an oscillator. When the oscillator reaches a certain point in its cycle, the switch turns on and the battery conducts a constant value of current of approximately 2 amperes at approximately 100 Hz. This test current is independent of the value of the internal resistance of the battery and will not change as the battery is used and its resistance increases. The remainder of the circuit measures the amplitude of the voltage across the battery terminals pursuant to the switching action of the oscillator. The voltage amplitude of the periodic signal is directly proportional to the internal resistance in the battery, and increased internal resistance results in increased voltage measured by the circuit. Hence it is the magnitude of the internal resistance that is the fundamental property whereby this invention monitors the battery""s current capability.
Current capability of a battery is inversely proportional to its internal resistance. This invention teaches how to use integration circuitry to obtain a signal indicative of the inverse of resistance. This improvement over U.S. Pat. No. 4,719,428 represents a fundamental change in testing the battery. That patent provides a voltage value indicative of battery voltage that drops while a large current is drawn as the battery is used. That drop is directly proportional to resistance and hence the net battery voltage which diminishes with use, is a linear function of resistance. By contrast, this invention provides a current capability that also diminishes with usage, but is a parabolic function of internal resistance.